We're not sure why all of the killer lakes of the world seem to be located in Africa, but here's another one to add to the list. Lake Nyos in Cameroon, West Africa, contains a large amount of carbon dioxide within its depths. For many years, no harm was done, until 1986 when the carbon dioxide exploded upwards. This caused a huge carbon dioxide 'cloud' to form and it resulted in extreme destruction and death.
The cloud traveled downhill and effectively suffocated 1,700 people and many more animals. Basically, nothing survived for 15 miles. Today, things aren't as precarious as they once were, as pipes exist to siphon the carbon dioxide from the lake. The risk still hasn't been completely eradicated, but for the most part it's considered safe. We still wouldn't be booking our flights in a hurry.